Many optical communication systems use dense wavelength division multiplexing (DWDM) in the wavelength region 1530-1565 nm, which represents the C-band gain window of erbium doped fiber amplifiers (EDFA). Because of the high optical gain of EDFA, when signals are transmitted simultaneously at closely spaced wavelengths, nonlinear effects such as Four Wave Mixing (FWM) arise in these systems which impose serious limitations, especially when operated very close to the zero dispersion wavelength (ZDW). To overcome this difficulty, some communication systems use small dispersion fibers that have dispersions typically in the range of 2-8 ps/(km.nm). When using fibers with such dispersions, the phase matching condition is not satisfied and hence the effect of FWM could be greatly reduced.
Thus, by implementing DWDM in a small dispersion fiber and using EDFA and distributed feed-back (DFB) laser diodes as light sources, a very high bit rate over a few hundred kilometers can be obtained without using a repeater. However, if repeaterless transmission of signals over very large distances (e.g., greater than 1000 km) is desired, the residual dispersion (2-8 ps/(km.nm)) in these fibers will continue to accumulate and therefore will limit the number of bits that can be sent at each wavelength.
The present invention greatly reduces some of the difficulties encountered in the aforementioned optical communications systems. In particular, the present invention implements an optical fiber link including a small residual dispersion fiber (SRDF) connected to a dispersion compensating fiber (DCF). The dispersion slope of the SRDF is positive and that of the DCF is negative. The dispersion slope of the DCF is appropriately tailored to the dispersion slope of the SRDF so that the DCF compensates for the accumulated dispersion in the SRDF for all wavelengths spanning the entire C-band of an EDFA.
The DCF compensates the accumulated dispersion in the SRDF for wavelengths in the range 1530 nm to 1565 nm. The effective dispersion resulting from the combination of the DCF and the SRDF is less than about 0.08 ps/(km.nm), and the slope of the effective dispersion is less than about 0.016 ps/(km.nm2).
The SRDF typically includes an inner core layer, an outer core layer positioned coaxially with and formed on the outside of the inner core layer, and a cladding layer positioned coaxially with and formed on the outside of the outer core layer. In one embodiment, the inner core layer has a radius of about 2 xcexcm and an index of refraction of about 1.46017 at a wavelength of about 1550 nm. The outer core layer has an inner radius equal to the outer radius of the inner core layer and an outer radius of about 3.7 xcexcm, and an index of refraction of about 1.45057 at a wavelength of about 1550 nm. The outer cladding layer has an inner radius equal to the outer radius of the outer core layer, and an index of refraction of about 1.44402 at a wavelength of about 1550 nm.
In certain embodiments, the inner core layer, the outer core layer, and the cladding layer of the SRDF are made from SiO2 (silica). The inner core layer is doped with GeO2 having a mole fraction of about 10.3%, and the outer core layer is also doped with GeO2 with a mole fraction of about 4.3%. As such, the relative index of refraction difference between the inner core layer and the cladding layer is about 0.011, and the relative index of refraction difference between the outer core layer and the cladding layer is about 0.0045 so that the dispersion of the SRDF varies from about 6 ps/(km.nm) to about 8 ps/(km.nm) in the C-band.
In these embodiments as well as others, the DCF typically includes an inner core layer, an inner cladding layer positioned coaxially with and formed on the outside of the inner core layer, an outer core layer positioned coaxially with and formed on the outside of the inner cladding layer, and an outer cladding layer positioned coaxially with the outer core layer and formed on the outside of that layer. In one embodiment, the inner core layer has a radius of about 1.1 xcexcm and an index of refraction of about 1.47380 at a wavelength of about 1550 nm. The inner cladding layer has an inner radius corresponding to the outer radius of the inner core layer and an outer radius of about 5.33 xcexcm, and an index of refraction of about 1.43828 at a wavelength of about 1550 nm. The outer core layer has an inner radius equal to the outer radius of the inner cladding layer and an outer radius of about 7.7 xcexcm, and an index of refraction of about 1.44838 at a wavelength of about 1550 nm. The outer cladding layer has an inner radius equal to the outer radius of the outer core layer, and an index of refraction of about 1.44402 at a wavelength of about 1550 nm.
In some embodiments of the DCF, the inner core layer, the inner cladding layer, the outer core layer, and the outer cladding layer are all made from SiO2. The inner core layer and the outer core layer are doped with GeO2. The mole fraction of the GeO2 in the inner core layer and the outer core layer is about 18.4%, and about 2.9%, respectively. The inner cladding layer is doped with fluorine with a mole fraction of about 1.26%. For a DCF made with these materials, the relative index of refraction difference between the inner core layer and the outer cladding layer is about 0.02, the relative index of refraction difference between the inner cladding layer and the outer cladding layer is about xe2x88x920.004, and the relative index of refraction difference between the outer core layer and the outer cladding layer is about 0.003 so that the dispersion of the DCF varies from about xe2x88x92220 ps/(km.nm) to about xe2x88x92300 ps/(km.nm) in the C-band.
In particular embodiments of this aspect, the length of the SRDF is about 37 times the length of the DCF. Related aspects of the invention include the DCF itself which can be connected to the SRDF, a method of making the optical fiber link, and a method of using the fiber link to compensate for accumulated dispersion in the SRDF.
Among other advantages, the maximum value of the effective dispersion, DE, of the fiber link is sufficiently small so that an optical system, which implements the present invention, can use erbium doped fiber amplifiers (EDFA) and DFB laser diodes as the light sources to provide repeaterless transmission of signals simultaneously over several wavelengths for distances greater than 1000 km. Still further aspects, features, and advantages follow.